Short-term Prediction of Ionospheric TEC Based on ARIMA Model

In order to achieve high short-term prediction accuracy of ionospheric TEC,first,we transform a seasonal time series for ionospheric Total Electron Content (TEC) into a stationary time series by seasonal differences and regular differences with a full consideration of the Multiplicative Seasonal model.Next,we use the Autoregressive Integrated Moving Average (ARIMA) model taken from time series analysis theory for modeling the stationary TEC values to predict the TEC series.Using TEC data from 2008 to 2012 provided by the Center for Orbit Determination in Europe (CODE) as sample data,we analyzed the precision of this method for prediction of ionospheric TEC values which vary from high to low latitudes during both quiet and active ionospheric periods.The effect of the TEC sample’s length on the predicted accuracy is analyzed,too.Results from numerical experiments show that during the ionospheric quiet period the average relative prediction accuracy for a six day time span reaches up to 83.3% with average prediction residual errors of about 0.18±1.9 TECu.During ionospheric active periods it changes to 86.6% with an average prediction residual error of about 0.69±2.6 TECu.For the quiet periods,above 90% of predicted residual is less than ±3 TECu while during active periods,it is only about 81%.The two periods show that that the higher the latitude,the higher the absolute precision,and the lower the predicted relative accuracy.In addition,the results show that prediction accuracy will improve with an increase of the TEC sample sequences length,but it will gradually reduce if the length exceeds the optimal length,about 30 days.On the other hand,with the same TEC sample,as the predicted days increase,the predictive accuracy decreases.Athough the predictive accuracy is not apparent at the beginning,it will be significantly reduced after 30 days.

Identification of weathered troughs in granites for tunneling at Daya Bay reactor-neutrino experiment site

Weathered troughs are frequently encountered in granites. They can cause problems to tunneling in the rocks and have to be properly addressed. The structures or spatial shapes of weathered troughs were seldom clarified in the past. In this paper, four weathered troughs are identified by means of geophysical exploration, core drilling, logging, and in-situ stress measurement at Daya Bay reactor-neutrino experiment site in Shenzhen, China. The weathered troughs are exposed on the ground or partially covered by grass and soils, which will threaten the safety of horizontal tunneling at a shallow depth. High electrical resistivity （HER） method is adopted for its feasibility and practicality, in combination with field geological observation, ultrasonic televiewer in boreholes and in-situ stress measurement. By comparing the HER values of completely decomposed to fresh rocks, it is indicated that the HER values of weathered troughs in natural state are 0.4-100 D.m, and the in-situ stress is abnormally lower than those at upper and lower layers. Field investigations show that the depths of the four weathered troughs are 30-182 m, with bottom elevation over 10 m. The volume of each weathered trough is mostly over lx l 0^6 m3 in inverted conic form. The weathered troughs often occur in various kinds of landforms, such as ridges, gullies or gently dipping dish-like depression areas. Faults and boundaries of different granitic plutons as well as joints govern the formation, locations and strikes of these troughs under development.